This application claims the priority of German patent document 102 39 017.7, filed Aug. 20, 2002 (PCT International Application No. PCT/EP2003/009085), the disclosure(s) of which is (are) expressly incorporated by reference herein.
The present invention relates to a method of measuring stress/strain by means of Barkhausen noise, such as may be used, for example, to measure stress/strain in bolts or rivets at the wing or fuselage of an airplane, particularly during maintenance work.
For example, for tightening bolts, torque wrenches have been used, while simultaneously determining the internal stress in the bolt by means of ultrasonic measurements. However, this technique requires that the bolt be coated or laminated with a piezoelectric, which makes it considerably more expensive.
In addition, it is known to use micromagnetic methods and sensors which measure Barkhausen noise to detect material changes caused by treatment processes, based on the proposition that the magnetic structure of the materials is influenced by the characteristics of the material. Crystallite limits and other inhomogeneities (such as dislocations, foreign atoms and inclusions) hinder the movement of the so-called Bloch walls; and detachment of the walls from inhomogeneities results in “bounces” in the magnetization, or so-called “Barkhausen noise”. These jolt-type movements of the Bloch walls can be recorded in the form of brief electric pulses, using a coil. The number, height and intensity of the pulses depends on the material and its condition.
Such a method for the nondestructive evaluation of ferromagnetic materials is known, for example, from German Patent Document DE 196 31 311 C2. The measuring principle is based on the fact that, when a ferromagnet is periodically reversed, the magnetic domain structure is continuously changed, and boundaries between areas of the same magnetization (that is, Bloch walls) move through the material structure and interact with the microstructure of the material. This interaction is received as an electromagnetic signal—the so-called Barkhausen noise.
Such methods, which are based on an analysis of the Barkhausen noise, are generally used for quality control; for example, for optimizing different treatment processes (grinding, heat treatment, etc.) of components. The components may, for example, be ground parts, camshafts, crankshafts, bearings, gear wheels, injection valves and numerous other parts from automotive engineering and the aerospace field.
One object of the invention is to provide a fast and simple method of measuring stress/strain.
Another object of the invention is to provide such a method which is capable of determining stress/strain conditions in fastening devices.
These and other objects and advantages are achieved by the method according to the invention, in which an exciting/sensing device is arranged at least adjacent to one magnetic or magnetizable element (preferably in a partial area around the magnetic or magnetizable element), so that it is acted upon by a rising magnetizing current. The sensing device thus detects the starting of the Barkhausen noise in the element (which is a measurement of the stress/strain condition of the element) as a function of the magnetizing current. Expediently, the starting of the Barkhausen noise is determined by a comparative measurement relative to reference values.
The invention has the advantage that the physical effect of the Barkhausen noise is effectively utilized. The stress/strain condition of a magnetic or magnetizable element (preferably ferromagnetic) is determined in a contactless manner by a simple mounting of an exciting/sensing device. In this case, the element is magnetized by the magnetic field generated by the exciting device, and the sensing device detects the Barkhausen noise.
In a preferred embodiment, the exciting/sensing device is constructed in one piece (preferably a single coil), which acts simultaneously as an exciting device and a sensing device. The sensing device detects the magnetizing current at which the Barkhausen noise occurs, the magnetizing current being proportional to the internal stress in the element. Such an arrangement has the advantage that the equipment expenditures are kept as low as possible.
In an alternative embodiment, the exciting device is again constructed as a coil, but the sensing device for detecting the Barkhausen noise is an acoustic or interferometric detector, which is notable for its small dimensions.
A pulsed magnetizing current is expediently used for excitation, and during the off-time of the pulses, the sensing device is set to receive the Barkhausen noise. This arrangement has the advantage that higher magnetic fields can be generated without thermally overloading the exciting device.
According to another preferred embodiment, an intermediate element of a non-magnetic or non-magnetizable material (e.g., a washer or the like) is arranged between the magnetic or magnetizable element (for example, a fastening device in the form of a bolt) and a structure, such a component, which is to be connected therewith. This embodiment has the advantage that it measures directly the stress existing in the fastening device as a result of the fastening. The washer is optional and preferably consists of a non-ferromagnetic material.
In an alternative embodiment, a magnetic or magnetizable element is first arranged between a non-magnetic or non-magnetizable fastening device and a structure to be connected therewith. Advantageously, stress/strain conditions of a non-magnetic fastening device can therefore also be determined by measuring the mechanical stress that is transmitted by the fastening device to a preferably ferro-magnetic element (for example, a washer).
The method according to the invention is preferably used when measuring stress/strain conditions of fastening devices (such as screwed or inserted bolts, rivets, etc.). The method can be used in multiple manners, for example, in the maintenance of airplanes, helicopters, motor vehicles, etc.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.